Holdout not lining up properly

Hi guys,

I am having trouble with a holdout in my holdout. I’m plugging the geometry straight into the mesh pipe of the Holdout node which then goes into the Render node. But I’m getting weird gaps when rendering on top of the preview render (which does line up with the geo).
Here is a pic:

Any ideas what might be causing this?
To generate the vdb I am using a scaled and transformed version of the geo to manipulate the velocity guide, but that shouldn’t have anything to do with the holdout, right?!

Any ideas what might be causing this?


Weird, could it be colliders and not holdout maybe?

Happy to have a look at the shot if you want to send it over? ( I probably have access to the crab gdrive data to save you uploading)


Hi Frank,

I am not really sure I understand the setup completely. You lost me a bit when you mentioned the vdb and how that relates to the rendered image. Do you mind elaborating a bit more ?


Hi guys,
sorry for the delay, I didn’t get the usual email notification (user error).
I’ll try to clarify:
I have a static object that is moving just above the ground and is supposed to kick up some silt.
Since the object isn’t actually touching the ground, I moved the mesh down a bit so that it intersects the ground, then used that intersection as the initial emitter.
I then scaled up the translated SDF by subtracting a constant value to have a larger and lower volume than what the original mesh is. That larger volume is then used to multiply an upward vector field which is kicking up the silt.
Here is a screen shot:

I then use the original mesh as a collider for the sim, and then later as a holdout in the renderer, and that is when I see the strange offset.

Does that make sense?
I will have a play with default geometry today to try and narrow this down a bit. If I can’t solve it I will post the script here later today.

Thanks guys,

So here is a generic example with just a sphere and a cube for the ground:
If you check, say, frame 8 you can see the strange volumetric shadow that looks like a misaligned holdout:

Edit: When I render deep without a holdout a DeepMerge will yield the expected result.

having a look now Frank.
It looks to me like it’s a shadow of the holdout cast from the default lighting rig.
The alpha im getting here looks ok.

Try adding a environment light , or quad light above, and see if that helps.


Ah, that would indeed explain it. I wasn’t aware that there was any shadowing happening from the holdout mesh.

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